There is currently considerable interest and research into ways to convert solar energy to electrical energy to help overcome the present society's dependence on a diminishing supply of fossil fuels and the problem-ridden nuclear fuels. Almost 50% of oil refined in the United States is imported and it is becoming increasingly important to utilize other energy sources to satisfy our technological society's need for electrical energy.
The chemical literature contains numerous references to consideration of hydrogen as an energy carrier. Producing hydrogen from water affords a means of energy conversion by which thermal energy from a primary source, such as solar or nuclear fusion or fission, can be changed into an easily transportable and ecologically acceptable fuel and was a subject considered by Dr. James E. Funk, Conference Proceedings--1st World Hydrogen Energy Conference, Miami Beach, Fla., Mar. 1-3, 1976. Dr. Melvin G. Bowman at the same conference disclosed aqueous iron-chloride cycles for hydrogen production.
Other art in the area utilizes metallic salts. G. R. B. Elliot, Conference on Thermodynamics and National Energy Problems (AD/A-012 702), June 1974, disclosed electrochemical heat engines wherein energy is produced by charging under conditions of low voltage and discharging under conditions of high voltage. The only electrochemical reaction mentioned was: EQU K.sub.2 S(FS).revreaction.2K(g)+1/2 S.sub.2 (g)
where (FS) refers to fused salt and (g) refers to gas phase. In a 1977 paper of the proceedings of the Symposium in High Temperature Metal Halide Chemistry (Electrochemical Society), Vol. 78-1, G. R. B. Elliot disclosed an electrochemical heat engine employing the electrochemical reaction EQU 2Li(1)+I.sub.2 (g).revreaction.2LiI(FS)
where (1) refers to liquid phase.
Additional art in the area discloses aqueous iron chlorides. For example, U.S. Pat. No. 3,553,017 discloses electrochemical power units, more particularly storage batteries, utilizing the electrochemical reaction EQU 1.5FeCl.sub.2 (aq).revreaction.0.5Fe(s)+FeCl.sub.3 (aq)
where (aq) refers to an aqueous solution and (s) refers to solid phase. This is an aqueous system not involving gaseous FeCl.sub.3 as a product which is necessary in the present invention. M. Warshay and L. D. Wright, J. Electrochem. Soc. 124, 173 (1977) discuss cost and size estimates for electrochemical bulk energy storage for use by electrical utilities via the aqueous reaction: EQU TiCl.sub.4 (aq)+FeCl.sub.2 (aq).revreaction.TiCl.sub.3 (aq)+FeCl.sub.3 (aq)
German Pat. No. 2,605,899 discloses a storage battery probably utilizing the following reaction: EQU Fe(AlCl.sub.4).sub.2 (FS)+NaAlCl.sub.4 (FS).revreaction.Na(1)+FeCl.sub.3 (FS)+3AlCl.sub.3 (FS)
where (FS) refers to fused salt chloride melt. Again, this patent does not teach gaseous FeCl.sub.3, an essential product of the present invention.
U.S. Pat. No. 3,374,120 discloses decomposition of fused salts, including iron halides, into their elemental constituents as a basis for the generation of electrical energy from thermal energy.
U.S. Pat. No. 4,064,325 discloses electric storage batteries utilizing a molten alkali metal negative electrode and an iron III/iron II or iron III/iron metal halide redox couple at the positive electrode, the nature and proportions of said materials being chosen to insure that a liquid state is maintained.
None of the prior art discloses electrochemical heat engine processes for the conversion of thermal energy, e.g., solar energy, to stored electrochemical energy and subsequently to electrical energy utilizing the chemical reactions of the present invention nor the apparatus that will be described in detail below. In addition, the present invention provides long-term storage of electrochemical energy and utilizes readily available and economical materials. Also, the oxidant product, gaseous FeCl.sub.3, results in an unexpectedly large decrease in the free energy change for the electrochemical reaction of the cell, upon charging, per 100.degree. C. rise in temperature. Furthermore, mobility may be imparted to both redox products, for example, Fe(s) and FeCl.sub.3 (g). Fe(s) is magnetic and easily moved by means of magnetic fields and FeCl.sub.3 is volatile above 316.degree. C. and hence can be removed as gas from solution above this temperature.